The aim of the proposed research is to study the influence of the immunological system on the development of malignancy. The chicken has been used as the experimental animal because it offers a unique model to study the influence of cellular and humoral immunity separately by manipulating the thymus and the bursa of Fabricius. Thymectomy experiments are designed to evaluate the possible role of cellular immunity in immune surveillance, and bursectomy to study the role of antibody mediated control mechanisms. The tumor systems used in this proposed research are reticuloendotheliosis virus (REV) strain T, Rous sarcoma virus transformed rat tumor cell line XC, and a benzo(a)pyrene-induced transplantable tumor cell line BP-1. These tumors will be inoculated into intact, thymectomized, and bursectomized chickens to evaluate their growth rate in immunosuppressed animals, and to determine the involvement of thymus and/or bursa in tumorigenesis. BIBLIOGRAPHIC REFERENCES: Lam, K. M. and Linna, T. J. Effect of thymectomy and bursectomy on XC cell-induced tumors in chickens. Fed. Proc., 34:852, 1975. Linna, T. J. Increased growth rate of a benzo(a) pyrene-induced transplantable tumors in bursectomized chickens. Fed. Proc. 34:963, 1975.